This invention relates to magnetic resonance (MR) techniques. More specifically, this invention relates to the compensation of magnetic fields to overcome distortions caused by the application of pulsed magnetic field gradients. The invention is particularly applicable to magnetic resonance imaging, but is not limited thereto.
The nuclear magnetic resonance (NMR) phenomenon has been utilized in the past in high resolution magnetic resonance spectroscopy instruments by structural chemists to analyze the structure of chemical compositions. More recently, MR has been developed as a medical diagnostic modality having applications in imaging the anatomy, as well as in performing in vivo, noninvasive spectroscopic analysis. As is now well known, the MR phenomenon can be excited within a sample object, such as a human patient, positioned in a homogeneous polarizing magnetic field, B.sub.o, by irradiating the object with radio frequency (RF) energy at the Larmor frequency. In medical diagnostic applications, this is typically accomplished by positioning the patient to be examined in the field of an RF coil having a cylindrical geometry, and energizing the RF coil with an RF power amplifier. Upon cessation of the RF excitation, the same or different RF coil is used to detect the MR signals, frequently in the form of spin echoes, emanating from the patient lying within the field of the RF coil. In the course of a complete MR scan, a plurality of MR signals are typically observed. The signals are used to derive MR imaging or spectroscopic information about the object studied.
The application of magnetic resonance to imaging, and to many of the techniques of localized spectroscopy, depend upon the use of magnetic field gradients to selectively excite particular regions and to encode spatial information within the NMR signal. During the NMR experiments, magnetic field gradient waveforms with particularly chosen temporal variations are used. Any departure from the application of ideal magnetic field gradient waveforms can, therefore, be expected to introduce image distortion. For example, imperfect rephasing of the nuclear spins and an attendant loss of signal occurs if the magnetic field gradients are not constant during selective time reversal pulses (i.e. use of 180.degree. time reversal RF pulses). This effect compounds in later spin echoes of multi-echo (Carr-Purcell-Meiboom-Gill) sequences. In addition, if the gradient field is not zero when it should be (due to residual decay after termination of a gradient pulse), the unintended phase dispersion can result in distorted spectra in chemical shift imaging (CSI) sequences as well as inaccurate spin-spin relaxation time (T.sub.2) determination in multi-echo sequences. Those skilled in the art are thus concerned particularly about the accuracy with which time varying magnetic field gradients are produced.
Distortions in the production of magnetic field gradients can arise if the gradient fields couple to lossy structures within the polarizing magnet such as its cryostat (if the magnet is of the superconductive design), or the shim coil system, or the RF shield used to decouple the gradient coils from the RF coil. The gradient distortions derive from induction of currents in these ambient structures, from the loss of energy to the shim coils or from an active response of the shim power supplies to the sensed gradient field. One observes, typically, an approximately exponential rise and decay of the magnetic field gradient during and after, respectively, the application of a rectangular current pulse to the gradient coil.
In pending U.S. patent application Ser. No. 816,074, now U.S. Pat. No. 4,698,591 which was filed on Jan. 3, 1986, and which is entitled "A Method for Magnetic Field Gradient Eddy Current Compensation," a method is disclosed which uses an analog preemphasis filter in the gradient power supply to shape the current applied to the gradient coil in such a way that the gradient field distortions are reduced. The filter includes a number of exponential decay components and adjustable potentiometers which must be set during system calibration. A measurement technique is used prior to system calibration in which the impulse response of the uncorrected magnetic field gradient is measured and the potentiometer settings for the pre-emphasis filter are then calculated.
It has been discovered that while such compensation of the magnetic field gradients improves performance of MR systems, distortions still arise as a result of the application of pulsed magnetic field gradients. More specifically, measurements indicate that eddy currents which are induced by magnetic field gradient pulses not only produce an unwanted magnetic field gradient field, but also cause temporal variations in the spatially homogeneous polarizing magnetic field B.sub.o. That is, magnetic field gradient pulses cause spurious changes in the magnitude of the polarizing magnetic field B.sub.o. The degree to which these distortions occur depends to a certain extent on the construction of the magnet and coil. For example, accurate alignment of the gradient coils with respect to the magnet structure reduces the distortion. In addition, the degree to which these distortions affect the results depends on the nature of the particular MR measurement being conducted. For example, MR scans in which the imaging magnetic field gradient pulses are changed in a nonmonotonic manner are affected more than conventional scans.